1. Field of the Invention
The present invention relates to a substrate for optical recording media of a so-called land-groove recording type, an optical recording medium, a manufacturing process for optical recording media, and an optical recording/reproducing method. More particularly, this invention is intended to minimize thermal crosstalk between adjoining tracks, that is, between a land and an adjoining groove, minimize a noise, and thus improve a recording density.
2. Description of the Related Art
With the recent trend to digitization of information, there is an increasing demand for media in which a large amount of digital data can be recorded. Media capable of meeting the demand include a so-called phase change optical recording medium, for example, a phase change optical disk, and a magnetooptical disk. The phase change optical recording medium has crystallinity thereof thermally changed to change the optical characteristic thereof, whereby recording is achieved. The magnetooptical disk has a magnetooptical effect thereof changed with application of heat and a magnetic field.
In general, the phase change optical disk or magnetooptical disk has, as seen from a schematic sectional view of FIG. 9 showing a major portion of the disk, a recording layer 5 formed over a substrate 11 for optical recording media with a transparent dielectric layer 51 between them. The substrate 11 transmits light. The transparent dielectric layer 51 is made of, for example, SiN or ZnS-SiO2. The recording layer 5 is a magnetooptical recording layer or a phase change recording layer and so on.
The phase change optical disk is such that changes in reflectance for predetermined light are detected in order to reproduce a recorded information signal. The magnetooptical disk is such that changes in polarization of predetermined light are detected in order to reproduce an information signal.
In these optical disks, guide grooves, that is, groove-like concave parts 2 that are generally referred to as grooves are formed spirally or concentrically on the surface of a substrate of each disk. The concave parts 2 are used to route a laser beam, which is emitted from an optical pickup of a recording/reproducing apparatus, along a train of information areas, or in other words, to produce a tracking servo signal.
For achieving high-density recording, a so-called land-groove recording method is adopted in order to record an information signal both in a groove or in a convex part 3 between the adjoining grooves, or a so-called land.
A technique for recording an information signal in the phase change optical disk or magnetooptical disk is a so-called thermal recording method in which laser light is converged in order to raise the temperature of the recording layer.
In this case, the foregoing land-groove recording method is adopted for attaining a larger recording density. In addition, a cycle of creating a groove, that is, a pitch between a concave part and an adjoining convex part must be minimized.
However, when the cycle of creating a groove is shortened, recording or reproducing becomes more susceptible to thermal interference such as thermal crosstalk, cross-writing, or cross-erasure occurring between adjoining tracks, that is, a groove and an adjoining land. Consequently, a record signal may be undesirably erased or vanished, and a reproduced signal may be deteriorated or faded away. This hinders realization of a high recording density.
For overcoming the above disadvantage, Japanese laid-open patent publication No. 11-25534 has disclosed a magnetooptical disk.
The magnetooptical disk is structured so that a border wall between a groove and a land is an inclined plane and has a width of 160 nm or more. Owing to this structure, a substantial distance between the groove and land is increased in order to minimize the thermal crosstalk.
The modulation degree of a push-pull signal is maximized when the difference D in height between a land and groove is approximately (2mxe2x88x921)xcex/(8n) (where m denotes a natural number that is 1, 2, 3, etc.). Herein, xcex denotes the wavelength of laser light used for reproduction, and n denotes the refractive index of a disk substrate. In general, for attaining a sufficiently large modulation degree of a push-pull signal together with a large difference in height D, the condition of (2mxe2x88x921)xcex/(8n) less than D less than (2mxe2x88x921)xcex/(5.5n) should be satisfied.
The critical frequency of a record signal detectable from an optical disk is proportional to xcex/NA where xcex denotes the wavelength of laser light and NA denotes the numerical aperture of an objective lens. For attaining a larger recording density, the wavelength of laser light should be reduced and the numerical aperture NA of the objective lens should be increased.
In recent years, the wavelength of laser light emanating from a solid-state laser is converted by using a second harmonic generation element (SHG) and the light source whose wavelength xcex falls below 415 nm have been obtained. Furthermore, owing to an advancement of semiconductor laser technologies, the semiconductor laser can produce laser light whose wavelength is about 407 nm.
For example, assume that the wavelength of laser light is 407 nm and the refractive index n of a substrate is 1.55. In this case, for maximizing the modulation degree of a push-pull signal used to extend tracking servo control, the difference in height D between a land (convex part) and an adjoining groove (concave part) should be set to approximately 33 nm, 99 nm, 164 nm, or the like under the aforesaid condition. However, a more intense effect of suppressing thermal crosstalk is required to attain a high recording density. The difference in height D must therefore be set to the largest possible value.
However, when the difference in height D between a land and an adjoining groove is too large, it becomes hard to trace the surface shape of the substrate 11 because of the thicknesses of a transparent dielectric layer 51 and a recording layer 5 formed on the substrate. Assuming that the transparent dielectric layer and recording layer are formed on the substrate through sputtering, a layer is deposited even on a sidewall linking a concave part and an adjoining convex part. This diminishes the width of the concave part. Besides, the deposition of the layer on the sidewall diminishes the number of sputtered particles reaching the concave part. Consequently, a layer formed in the concave part becomes thinner.
The heat capacity of the recording layer differs between a concave part and an adjoining convex part. This leads to a difference in sensitivity between a groove and an adjoining land and causes inhomogeneity of an optical disk.
An object of the present invention is to provide a substrate for optical recording media, an optical recording medium, a manufacturing process for optical recording media, and an optical recording/reproducing method. Herein, a so-called land-groove recording method in which data is recorded in both a land and an adjoining groove is adopted in an effort to improve a recording density. Thermal crosstalk between the land and adjoining groove can be suppressed effectively. Moreover, inhomogeneity of an optical recording medium between the land and adjoining groove can be suppressed effectively.
According to the present invention, what is referred to as the optical recording/reproducing method includes a method of performing at least one of optical recording and optical reproducing.
Accordingly, the present invention is concerned with land-groove recording.
According to the present invention, a substrate for optical recording media has groove-like concave parts created on at least one surface thereof, and has convex parts each created between adjoining concave parts. A border sidewall linking a concave part and an adjoining convex part has at least a first sidewall plane adjoining the bed of the concave part and a second sidewall plane leading to the apical surface of the convex part.
The first sidewall plane meets the bed of the concave part at an angle xcex81 ranging from 120xc2x0 to less than 180xc2x0. The second sidewall plane meets the bed of the concave part at an angle xcex82 ranging from 90xc2x0 to 110xc2x0.
Moreover, according to the present invention, in a substrate for optical recording media, a border sidewall linking a concave part and an adjoining convex part has at least a third sidewall plane and a fourth sidewall plane. The third sidewall plane adjoins the bed of the concave part. The fourth sidewall plane leads to the convex part while adjoining the first sidewall plane with an intermediate plane between them. The third and fourth sidewall planes meet the bed of the concave part at angles xcex83 and xcex84 ranging from 90xc2x0 to 150xc2x0. The intermediate plane and the fourth sidewall plane meet at an angle xcex85 ranging from 90xc2x0 to 150xc2x0.
Moreover, according to the present invention, an optical recording medium is structured so that a recording layer will be formed over a surface of the foregoing substrate for optical recording media in accordance with the present invention on which the concave and convex parts are created.
In particular, in an effort to further improve a recording density in the recording layer, record information is preferably recorded in the form of magnetic domains and the reproduction of the information is preferably carried out by at least one of expansion of the magnetic domains and shrinkaze thereof based on temperature distribution derived from irradiation of light.
Moreover, according to the present invention, a manufacturing process for optical recording media is characterized in that a discontinuous part of a recording layer is produced to extend in the depth direction of the concave part and transverse the border sidewall.
Moreover, according to the present invention, an optical recording/reproducing method is such that an optical recording medium having the foregoing structure is employed, and recording and/or reproducing, or in other words, at least one of recording and reproducing is performed on the optical recording medium through irradiation of light. The light to be irradiated is laser light whose wavelength ranges from 330 nm to about 660 nm.
Assume that an optical recording medium is made using the aforesaid substrate for optical recording media in accordance with the present invention. In this case, even if a recording layer, that is, an optical recording layer, to which light is irradiated for the purpose of recording and/or reproducing, is formed on the substrate, thermal crosstalk is effectively suppressed.
This is presumably attributable to the fact that the recording layer is formed to have discontinuous parts produced on both edges of each groove, that is, at a borderline between a groove and an adjoining land. In the discontinuous part, the heat conductivity of the recording layer deteriorates because of a change in the physical property or thickness thereof.